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Russian oil company Rosneft is buying a 30 percent share of 20 exploration blocks held by ExxonMobil in the deep waters of the Gulf of Mexico.

Exxon will retain a 70 percent stake in each block and remain the operator. Rosneft received an option to acquire minority stakes in some of Exxon's Gulf fields when the two signed an exploration agreement in Russia last year.

The agreement was considered a big score for Exxon. The Irving, Texas, company gained access to some of the world's richest sources of oil and other hydrocarbons in the Black Sea and the Russian Arctic.

Seismic data from the Gulf of Mexico fields is being analyzed, Exxon Mobil Corp. said Wednesday. The blocks aren't yet producing oil.